Doll Etienne V, Scherer Siegfried, Wenning Mareike
Chair of Microbial Ecology, Institute for Food and Health, Technische Universität München Freising, Germany.
Front Microbiol. 2017 Jan 31;8:135. doi: 10.3389/fmicb.2017.00135. eCollection 2017.
Premature spoilage and varying product quality due to microbial contamination still constitute major problems in the production of microfiltered and pasteurized extended shelf life (ESL) milk. Spoilage-associated bacteria may enter the product either as part of the raw milk microbiota or as recontaminants in the dairy plant. To identify spoilage-inducing bacteria and their routes of entry, we analyzed end products for their predominant microbiota as well as the prevalence and biodiversity of psychrotolerant spores in bulk tank milk. Process analyses were performed to determine the removal of psychrotolerant spores at each production step. To detect transmission and recontamination events, strain typing was conducted with isolates obtained from all process stages. Microbial counts in 287 ESL milk packages at the end of shelf life were highly diverse ranging from <1 to 7.9 log cfu/mL. In total, 15% of samples were spoiled. High G+C Gram-positive bacteria were the most abundant taxonomic group, but were responsible for only 31% of spoilage. In contrast, psychrotolerant spores were isolated from 55% of spoiled packages. In 90% of samples with pure cultures of sensu lato and spp., counts exceeded 6 log cfu/mL. In bulk tank milk, the concentration of psychrotolerant spores was low, accounting for merely 0.5 ± 0.8 MPN/mL. was by far the most dominant species in bulk tank milk (48% of all isolates), but was never detected in ESL milk, pointing to efficient removal during manufacturing. Six large-scale process analyses confirmed a high removal rate for psychrotolerant spores (reduction by nearly 4 log-units). sensu lato, on the contrary, was frequently found in spoiled end products, but was rarely detected in bulk tank milk. Due to low counts in bulk tank samples and efficient spore removal during production, we suggest that shelf life is influenced only to a minor extent by raw-milk-associated factors. In contrast, recontamination with spores, particularly from the complex, seems to occur. To enhance milk quality throughout the entire shelf life, improved plant sanitation and disinfection that target the elimination of spores are necessary.
由于微生物污染导致的过早变质和产品质量参差不齐，仍然是微滤和巴氏杀菌延长保质期（ESL）牛奶生产中的主要问题。与变质相关的细菌可能作为原料乳微生物群的一部分进入产品，也可能作为乳制品厂中的再污染物进入。为了识别导致变质的细菌及其进入途径，我们分析了最终产品中的主要微生物群，以及大容量储存罐牛奶中耐冷孢子的流行情况和生物多样性。进行了工艺分析，以确定每个生产步骤中耐冷孢子的去除情况。为了检测传播和再污染事件，对从所有工艺阶段获得的分离株进行了菌株分型。在保质期结束时，287个ESL牛奶包装中的微生物数量差异很大，范围从<1到7.9 log cfu/mL。总共有15%的样品变质。高G+C革兰氏阳性菌是最丰富的分类群，但仅导致31%的变质。相比之下，55%的变质包装中分离出了耐冷孢子。在90%的含有狭义嗜冷栖热放线菌和嗜热栖热放线菌纯培养物的样品中，菌数超过6 log cfu/mL。在大容量储存罐牛奶中，耐冷孢子的浓度很低，仅为0.5±0.8 MPN/mL。嗜热栖热放线菌是大容量储存罐牛奶中迄今为止最主要的物种（占所有分离株的48%），但在ESL牛奶中从未检测到，这表明在生产过程中得到了有效去除。六项大规模工艺分析证实了耐冷孢子的高去除率（降低了近4个对数单位）。相反，狭义嗜冷栖热放线菌在变质的最终产品中经常被发现，但在大容量储存罐牛奶中很少被检测到。由于大容量储存罐样品中的菌数较低，且生产过程中孢子去除效率较高，我们认为保质期仅在较小程度上受原料乳相关因素的影响。相比之下，似乎存在孢子再污染的情况，特别是来自嗜热栖热放线菌复合体的孢子。为了在整个保质期内提高牛奶质量，有必要改进针对孢子消除的工厂卫生和消毒措施。
